LIKELY&UNLIKELY 宏提示编译器做分支预测

#define LIKELY(x) (__builtin_expect((x), 1))
#define UNLIKELY(x) (__builtin_expect((x), 0))

#include <folly/likely.h>

int a = 1;
if (LIKELY(a == 1)) {
    a = 2;
}

if (UNLIKELY(a == 3)) {
    a = 3;
}